Presently, the bulk volume of the world production of photovoltaic elements comprising solar panels is based on multi-crystalline silicon wafers cut from ingots that are cast by directional solidification (DS) based on the Bridgeman method in electrically heated furnaces. The crucible being employed is usually made of silica, SiO2, and the furnaces have heating elements above, below and/or sidewise with respect to the crucible to provide the heat for melting and control of heat extraction during the directional solidification. The process may be described as follows:
A crucible open at the top made of SiO2 is covered in its interior with silicon nitride containing coating and filled with a silicon feedstock to a predetermined height. The crucible is then placed on the floor of the heating compartment of the furnace. Next, a circumferential support structure of graphite plates is attached along the outer crucible walls to provide mechanical support at elevated temperatures when the SiO2-crucible softens and tends to sag. The furnace compartment is then closed, evacuated, and inert purge gas is supplied during the period the heating elements are engaged in melting/solidification of the silicon feedstock. When the silicon is melted, the heating is adjusted to obtain a directional solidification. An inert purge gas, usually argon, is flushed onto the surface of the silicon to protect against gaseous contamination and to remove effectively SiO-gas at least as long as the silicon is in liquid phase.
A main challenge in these processes is to maintain the purity of the molten silicon material during melting and solidification. The melt is usually protected from gaseous contaminants by a combination of evacuating the atmosphere in the hot zone of the furnace and flushing a cover of inert purge gas over the surface of the liquid silicon phase. However, the amount of purge gas may be insufficient to prevent back flows of CO generated inside the furnace chamber (due to release of SiO from the melt with subsequent contact with graphite parts of the hot zone) resulting in formation of SiC impurities in the melt. Build-up of carbon in the silicon melt leads to formation of SiC precipitates responsible for shunting effects (short circuit of pn-junctions) in solar cells leading to drastic degradation of efficiencies of photovoltaic cells. Especially high amounts of CO are generated in cases where the furnace is subject to leakages of ambient air into the interior of the hot zone. Another drawback of low purge gas flow, is that it leads to reduced evaporation of SiO from the melt, and consequently to a reduction of cell efficiencies due to light induced degradation related to higher oxygen contents in the silicon. Meanwhile, utilization of higher amounts of gas flows leads to degradation of silicon nitride coating with subsequent sticking of silicon to silica crucible walls, which causes loss of ingots due to their cracking. Thus proper and adequate control of the gas flow poses a challenge for the crystallization process of silicon for photovoltaic industry.
Silicon carbide (SiC) inclusions lead to productions losses in the downstream sawing of wafers from the blocks cut from the solidified ingots, in that the SiC-particles cause wire breakage in sawing machines and/or form saw marks on the wafers. Thus, there is a need for controlling/eliminating the intrusion of carbon and obtaining sufficient evaporation of oxygen from the silicon metal during formation of the crystalline ingots.
Further, the SiO-gas will when contacted with carbon-containing elements of the furnace react with the carbon to form CO-gas and solid SiC. The CO-gas may enter the molten silicon as described above, and the SiC will form a solid deposited phase on the surface of the carbon-containing elements being contacted by the SiO-gas. The deposition of SiC is a problem since carbon-containing structural elements of the furnace are detrimentally affected by the build-up of SiC-deposits. This is especially a problem for the graphite insulation wall of the hot zone and heating elements made of graphite. The formation of SiC-deposits changes not only the electrical properties of the heating elements requiring their frequent replacement, but also deteriorates the thermal properties of graphite insulation of the hot zone. The deterioration of carbon-containing structural elements of the hot zone of the furnace represents a substantial cost increase due to reduced service time of the furnace elements and the interruption in production associated with the maintenance work.